Warning: This crate is still in the early stages of development, and is not guaranteed to be stable. Also, I built this for fun to use with my personal projects without having any idea about how stuff like this usually works, so until I eventually look into this topic more and find time for optimiziations, be warned that this implementation is likely not optimal.
sprout is a rust crate that parses text into ASTs (Abstract Syntax Trees), given definitions for tokens
and grammars built from those tokens.
First, define an enum for your tokens. It should derive the necessary traits shown below.
```rust
pub enum Token { Number, Word, Space } ```
Implement std::fmt::Display for your token enum. This will be used to generate human-readable error messages.
```rust use std::fmt;
impl fmt::Display for Token { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, match self { Self::Number => "number", Self::Word => "word", Self::Space => "space" }) } } ```
Now, you can use the alphabet macro to provide definitions for your tokens using a subset of regular expressions
syntax that includes parentheses (()), square brackets ([]), ranges ([a-z]), as well as the operators *, + and ?.
rust
let alphabet = alphabet! {
Token::Number => "[0-9]+";
Token::Word => "[a-z]+";
Token::Space => " "
}
Next, you define your grammar, in terms of "procedures", or "procs". They are the abstract parts of your language that will end up forming the nodes of your syntax tree.
First of all, you again create an enum:
```rust
pub enum Proc { TwoOrThreeWords, WordOrNumber, Sequence } ```
And, again, implement std::fmt::Display:
rust
impl fmt::Display for Proc {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, match self {
Self::TwoOrThreeWords => "two or three words",
Self::WordOrNumber => "word or number",
Self::Sequence => "word/number sequence"
})
}
}
Finally, define your procedures using the grammar macro in conjunction with token, proc, repeat, choice and option.
rust
let grammar = grammar! {
Proc::TwoOrThreeWords =>
token!(Token::Word),
token!(Token::Space),
token!(Token::Word),
option!(
token!(Token::Space),
token!(Token::Word)
);
Proc::WordOrNumber =>
choice!(
token!(Token::Word);
token!(Token::Number)
);
Proc::Sequence =>
repeat!(
choice!(
proc!(Proc::TwoOrThreeWords);
proc!(Proc::WordOrNumber)
)
);
}
As you can see, using proc, procedures can reference other procedures in the grammar, or even themselves.
Also: note precedence: The different paths in a choice will be checked in the order they appear, and including the content of an
option will always be preferred over exluding it.
Finally, from your alphabet and grammar, you can construct a parser:
rust
let parser = Parser::new(alphabet, grammar);
This parser will now simply spit out an AST (or a ParsingError) for any string you throw at it! For example this input
rust
let tree = parser.parse("abc ab 123 xyz 69");
would produce an AST like this:
Sequence "abc ab 123 xyz 69"
TwoOrThreeWords "abc ab"
WordOrNumber "123"
WordOrNumber "xyz"
WordOrNumber "69"
Specifically, the output format is a trees::Tree of ASTNodes, where ASTNode has these fields:
| Name | Type | Description |
|------|-----------------------------------------------|-------------------------------------------------------|
| proc | Proc (or whatever you called it) | The procedure that this node corresponds to |
| text | String | The text contained in the instance of the procedure |
| pos | TextPosition (has fields line and char) | The position of the start of the instance in the text |